1. Field of the Invention
The present invention relates to a efficient prism configurations for line-narrowing resonators of excimer or molecular fluorine laser systems beams having at desired spectral bandwidths.
2. Description of the Related Art
One application of the present invention is in the use of excimer lasers or molecular fluorine lasers as a light source for steppers and/or scanners for photographic microlithography in semiconductor manufacture. When such lasers are used for microlithography, it is desired to have laser emissions within a range which is much smaller than the natural laser linewidth of approximately 300 to 400 picometers (pm).
The extent of line narrowing desired for a particular application depends on the imaging optics of stepper/scanner devices. For refractive illumination optics, line-narrowing to less than 1 pm is desired, and between 10 pm and 100 pm bandwidths are desired for catadioptric or all-reflective illumination systems.
For use of eximer or molecular fluorine lasers in microlithography with catadioptric illumination systems a moderate narrowing of the laser bandwidth is sufficient. Examples of lasers that can be used in such applications are eximer lasers such as KrF lasers (emission wavelength approximately 248 nm) and ArF lasers (emission wavelength 193 nm) and molecular fluorine lasers emitting around 157 nm. Prisms are the line narrowing elements of choice for semi-narrow band laser resonators used with catadioptric illumination systems, since their losses are low and their durability is high in comparison to other elements like gratings.
Line narrowing for eximer or molecular fluorine lasers can be achieved by inserting prisms as wavelength dispersive elements in the resonator. The number of prisms and their arrangement determine the achievable bandwidth. Generally, the larger the number of prisms the narrower the bandwidth produced by the laser. Since the number of optical elements is directly correlated with losses in the resonator, the number of elements should be as low as possible in accordance with the bandwidth it is desired to achieve.
Previously dispersive prisms were Brewster prisms. These Brewster prisms are prisms where the apex angle is chosen in such a manner that the incident and exit angle are equal to the Brewster""s angle. These angles are chosen to be the Brewster angle because reflective loses are at a minimum at the Brewster""s angle. In the present invention, anti-reflective coatings allow incident angles to be greater than the Brewster angle while at the same time minimizing reflective losses.
It is desired to have a resonator arrangement including one or more dispersive prisms having optimized apex angles and beam entrance and exit surfaces, and oriented to optimize beam entrance and exit angles, to achieve more efficient bandwidth narrowing. It is also desired to configure optimum beam expansion prisms when used with a line selection element such as a dispersive prism, etalon or grating.
In accordance with any specific requirements for line narrowing, it is recognized in the invention that losses in the resonator may be minimized by minimizing the number of elements in the laser resonator. In order to minimize the number of elements in the resonator, it is further recognized in the invention that angles related to the prisms in the resonator, i.e., the angle of incidence, the angle of exit, the prism angle, and the apex angle of the prism, as well as surfaces of the prisms, may be advantageously optimized,
It is therefore an object of the invention to provide a line-narrowing resonator including one or beam expanding and/or dispersive prisms configured to achieve a desired narrowed bandwidth, while minimizing reflective losses at surfaces of the prisms, such as by minimizing the number of prism surfaces and/or by optimally configuring the prism surfaces, to achieve an optimal combination of line-narrowing and loss reduction in the resonator.
In a first aspect of the invention, in accord with the above object, a laser is provided, which is preferably an excimer or molecular fluorine laser, for generating an output having predetermined wavelength and bandwidth characteristics. The laser has a line-narrowing resonator including a dispersive prism, wherein the beam makes a non-symmetric pass through the prism, such that the angle of incidence at the entrance surface is different than the exit angle of the beam at the exit surface of the prism. One or both of these angles will thus be different than the Brewster angle and is preferably larger than the Brewster angle. An antireflective coating is preferably provided at one or both of the entrance and exit surfaces to minimize reflective losses. In a preferred embodiment, one or both of the entrance and exit angles is greater than 65xc2x0. The relationship between the entrance and exit angles is preferably selected, also according to a selected apex angle of the prism, for maximizing the dispersion of the prism.
In a second aspect of the invention, also in accord with the above object, a laser is provided, preferably an excimer or molecular fluorine laser, having a line-narrowing resonator including a dispersive prism, wherein the apex angle is larger than a prism wherein the apex angle is such that the angle of incidence of a beam at the entrance surface and the exit angle of the beam at the exit surface are each equal to Brewster""s angle. The apex angle is preferably greater than 65xc2x0, and may be greater than 75xc2x0. An antireflective coating is provided on one or both of the entrance and exit surfaces of the prism to minimize reflective losses.
The prisms preferably comprise CaF2, especially when the invention is incorporated with an ArF (193 nm) or F2 (157 nm) laser. One or more beam expanding prisms may be positioned in front of the dispersive prism. These beam expansion prisms have preferred apex angles between 37.5xc2x0 and 42.5xc2x0, and more preferably greater than 40xc2x0. The preferred beam expansion prisms also have an antireflective coating on one or both of beam entrance and exit surfaces.